1. Field
The present invention relates to broadcast communications, otherwise known as point-to-multipoint, in a wireline or a wireless communication system. More particularly, the present invention relates to a system and method for a handoff in such a broadcast communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211, issued on Jun. 3, 2003, assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
When a subscriber station travels outside the boundary of the base station with which the subscriber station currently communicates, it is desirable to maintain the communication link by transferring the call to a different subscriber station. The method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention. The method and system for providing a softer handoff is described in detail in U.S. Pat. No. 5,933,787, entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,” assigned to the assignee of the present invention. Using these methods, communication between the subscriber stations is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered a “soft” handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the subscriber unit is in communication with two base stations, the subscriber unit combines the signals received from each base station in the same manner that multipath signals from a common base station are combined.
In accordance with the above-cited inventions, each base station transmits a pilot signal of a common PN spreading code offset in code phase from pilot signals of other base stations. A subscriber station assisted soft handoff operates based on the pilot signal strength detected by the subscriber station. To streamline the process of searching for pilots, four distinct sets of pilot offsets are defined: the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set identifies the base station(s) or sector(s) through which the subscriber station is communicating. The Candidate Set identifies the base station(s) or sector(s) for which the pilots have been received at the subscriber station with sufficient signal strength to make them members of the Active Set, but have not been placed in the Active Set by the base station(s). The Neighbor Set identifies the base station(s) or sector(s), which are likely candidates for the establishment of communication with the subscriber station. The Remaining Set identifies the base station(s) or sector(s) having all other possible pilot offsets in the current system, excluding those pilot offsets currently in the Active, the Candidate and Neighbor sets.
The subscriber station is provided with a list of PN offsets corresponding to base stations of neighboring cells. In addition, the subscriber station is provided with a message which identifies at least one pilot corresponding to a base station to which the subscriber station is to communicate through. These lists are stored at the subscriber station as a Neighbor Set and an Active Set of pilots, and are updated as conditions change.
When communication is initially established, a subscriber unit communicates through a first base station and the Active Set contains only a pilot signal of the first base station. The subscriber unit monitors pilot signal strength of the base stations of the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. When a pilot signal of a base station in the Neighbor Set or Remaining Set exceeds a predetermined threshold level (T_ADD), the pilot signal identifier is added to the Candidate Set. The subscriber unit communicates a Power Strength Measurement Message (PSMM) to the first base station identifying the new base station. A system controller decides whether to establish communication between the new base station and the subscriber unit, and communicates the decision in a Handoff Direction Message (HDM). The message identifies the pilots of the Active Set which correspond to base stations through which the subscriber station is to communicate. The system controller also communicates information to each base station corresponding to a new pilot in the Active Set which instructs each of these base stations to establish communications with the subscriber station. The subscriber station communications are thus routed through all base stations identified by pilots in the subscriber station Active Set.
When the subscriber unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold (T_DROP) for a predetermined period of time (T_TDROP), the subscriber unit generates and transmits a message to report the event. The system controller receives this message through at least one of the base stations with which the subscriber unit is communicating. The system controller may then decide to terminate communications through the base station whose pilot signal strength as measured at the subscriber station is below the T_DROP.
The system controller upon deciding to terminate communications through a base station generates a new message identifying the pilots of the Active Set to which the subscriber station is to communicate through. In this message, which identifies pilots of the Active Set, the pilot of the base station to which communications with the subscriber station are to be terminated is not identified. The system controller also communicates information to the base station not identified in the Active Set to terminate communications with the subscriber station. The subscriber station, upon receiving the message identifying pilots of the Active Set, discontinues processing signals from the base station whose pilot is no longer in the Active Set. The subscriber station communications are thus routed only through base stations identified by pilots in the subscriber station Active Set. In the case where there were previously more than one pilot identified in the Active Set and now only one, the subscriber station communicates only to the one base station corresponding to the pilot identified in the subscriber station Active Set.
The above-described wireless communication service is an example of a point-to-point communication service. In contrast, broadcast services provide central station-to-multipoint communication service. The basic model of a broadcast system consists of a broadcast net of users served by one or more central stations, which transmit information with a certain contents, e.g., news, movies, sports events and the like to the users. Each broadcast net user's subscriber station monitors a common broadcast forward link signal. Because the central station fixedly determines the content, the users are generally not communicating back. Examples of common usage of broadcast services communication systems are TV broadcast, radio broadcast, and the like. Such communication systems are generally highly specialized purpose-build communication systems. With the recent, advancements in wireless cellular telephone systems there has been an interest of utilizing the existing infrastructure of the mainly point-to-point cellular telephone systems for broadcast services. (As used herein, the term “cellular” systems encompasses communication system utilizing both cellular and PCS frequencies.)
Although the described handoff method for subscriber units acting as point-to-point units described above could be applied to broadcast systems, because in a broadcast system, large number of subscribers monitor a common broadcast forward channel, a handoff based on base station-subscriber station signaling message exchange would result in a high signaling load. Furthermore, as described in the above-cited U.S. Pat. Nos. 5,267,261, and 5,933,787, the transmissions received simultaneously by a subscriber station during handoff are synchronized at the transmitting base stations. Because broadcast transmission is intended for many subscriber stations, the base station cannot synchronize transmission for each subscriber station desiring to handoff. Based on the foregoing, there is a need in the art for a system and method for handoff in such a broadcast communication system.